A known problem in the field of bedding is sheets being moved about, or even becoming completely separated from the bed, by body movement during sleep. Such movement can lead to sleep disruption or discomfort by way of bunching up of a loosened bottom sheet under the body, and other problems, such as direct exposure of the mattress to sweat when a loose bottom sheet is dislodged from beneath a sleeper. Fitted bed sheets, in particular, are manufactured to be used on a variety of mattress sizes that may vary somewhat in height, width and length. So too, a fitted bed sheet might vary slightly from its intended dimensions. As a result, the fitted bed sheet may have an excess amount of sheet slack that causes the fitted sheet to fit loosely on the mattress. Additionally, a flat bed sheet may be used to cover a mattress. The natural movement of a sleeping person can result in the bunching of the sheets and such an uneven surface can make it difficult to achieve a restful night sleep. Prior solutions to such bedding displacement issues have been proposed, including use of straps to secure or tie-down the fitted bottom sheet to the mattress to better resist movement thereof, and combining of a top sheet and fitted bottom sheet into a single unit to reduce top sheet movement by anchoring of the top sheet and fitted sheet together and onto the mattress. Examples of strap arrangements for securing bedding in place include those disclosed in U.S. Pat. Nos. 4,891,856, 6,161,235, 3,092,848, 4,040,133, 6,233,764 and U.S. Patent Application Publications 2009/0172881, 2009/0241261 and 2012/0060285. Elastic straps and bands are difficult to install due to the heavy weight of the mattress and may require two people to install; one person to lift the mattress while the other person stretches the straps or bands in place under the mattress. The elastic straps and bands often stretch too much and are unable to keep the sheet in place and tight to the mattress. Further, the clamps of the elastic straps or bands sometimes can slip off the sheet and damage the sheet. When tube-like clips with length-wise opening are used there are often problems with the installation process as wrapping the sheet around the pencil-sized seam of the mattress and covering the seam and sheet with the clip can be a difficult process. For at least these reasons there is a need for an improved sheet tightener that solves the problems above described.